1. Field of the Invention
The present invention generally relates to a compound semiconductor device and a method for forming the same, and more particularly, the present invention relates to a compound semiconductor device and a method for forming the same with regard to a semiconductor laser having a periodic corrugation.
This application is a counterpart of Japanese application Serial Number 120207/1997, filed Apr. 23, 1997, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
Distributed feedback (DFB) structure is an important approach toward the design of a mode selective optical cavity. The DFB approaches to create strong mode selectivity is based upon the propagation of waves in a periodic structure. We know that in periodic structures, special effects occur when the wavelength of the wave approaches the wavelength of the periodic structure. In semiconductor crystal, this leads to bandgaps and Bragg reflections. Similar effects occur for optical waves.
In the DFB structure, a periodic corrugation is incorporated into the laser structure. The fabrication process is by no means trivial and involves growing the basic laser structure, etching a periodic structure and regrowing the top layer. The corrugation should be as close as possible to an active layer so that the optical wave interacts strongly with the corrugation. However, since the placement of the corrugation creates defects, the corrugation cannot be too close to the active region. Accordingly, in the fabrication process of the DFB laser, it is very important whether or not an active layer can be formed on the corrugation without damaging the corrugation.